Core damage ahead of fight (6)
I believe the answer is:
marrow
'core' is the definition.
(I know that core can be written as marrow)
'damage ahead of fight' is the wordplay.
'damage' becomes 'mar' (to mar is to spoil or damage).
'ahead of' says to put letters next to each other (some letters go in front of others).
'fight' becomes 'row' (I've seen this in another clue).
'mar'+'row'='MARROW'
(Other definitions for marrow that I've seen before include "Stuff found inside bones" , "Large vegetable gourd or stuff in bones" , "Soft tissue in bones" , "one of two" , "Fatty stuff in bones or large fruit" .)